Various devices for internal fixation of bone segments in the human or animal body are known in the art. One type of system is a pedicle screw system, which is sometimes used as an adjunct to spinal fusion surgery, and which provides a means of anchoring an implantable member to a spinal segment. A conventional pedicle screw system comprises a pedicle screw and a rod-receiving device (also referred to herein as a coupling device or coupling assembly, since it couples a spinal rod to the pedicle screw or other bone anchor). The pedicle screw usually includes an externally threaded body or shank and an enlarged head portion, although the head portion may be eliminated by providing an integral shank portion extending from the rod-receiving device. The rod-receiving device often includes a top portion having a U-shaped channel to receive the spinal rod and a lower portion having a seat for receiving the head portion of the pedicle screw. Multiple screw assemblies may be implanted along the spine and connected by a rod to fix the vertebrae in a desired orientation and stabilize the spinal column. The goal of such a system may be, for instance, to substantially reduce and/or prevent relative motion between spinal segments that are being fused or to de-rotate an abnormal spine.
Some pedicle screw systems lack features that enhance and/or benefit newer, minimally invasive surgery (MIS) techniques that are more commonly being used for spinal surgeries. For instance, installation and locking of coupling assemblies often involves complex manipulation of a plurality of components of the screw system, and sometimes also requires a screw or other anchor to be assembled with a rod-receiving device prior to implantation. The assembly may obstruct the working space, making it more difficult to insert the anchor into a vertebra and manipulate the coupling device during surgery. Furthermore, assemblies with polyaxial fixation devices in the prior art ordinarily rely on downward force of the rod against the head of the bone anchor to secure the bone anchor against pivoting, so that the assembly, and specifically the rod receiving portion thereof, is provided with little support or stability prior to full locking of the rod. Some pedicle screw systems also include rather large and bulky assemblies to secure a rod, thus increasing opportunities for tissue damage in and around the surgical site during installation. Many of these systems also include set-screw type locking mechanisms or multi-part cap structures that require rotation or complex manipulation of small components and/or require a significant portion of the assembly to be located above the rod, increasing the height (profile) of the implants extending radially away from the spinal column, which may cause patient discomfort after implantation. Systems with set screws for securing the rod within the coupling assembly also lack a predetermined locking position, requiring a surgeon to turn the set screw a number of revolutions in order to secure the rod. When locking the rod in place with a set screw or other similar structure, the surgeon must also determine when the assembly “feels” locked, often resulting in overtorquing of the locking mechanism or false locking of the assembly. Cross-threading is also often a problem in such systems, and can result in damage to the assembly or a failure to fully lock the cap.
Due to the limited working space at the site of implantation and the number of components to be assembled, instruments have been developed to stabilize the coupling assembly, shift spinal rods into coupling assemblies, and lock rods in place by securing a locking cap to the coupling assembly. For instance, U.S. Patent Application No. 2006/0089651 discloses an instrument for advancing a spinal rod into a coupling member or yoke of a pedicle screw assembly. After a clamp device is used to secure the instrument to the coupling member, a drive assembly advances a locking cap and spinal rod into the coupling member in response to rotation of a first member, and then rotates the locking cap to secure the cap and rod to the coupling member in response to rotation of a second member.
Other such devices are disclosed in U.S. Patent Application No. 2003/0225408 and U.S. Pat. No. 6,648,888. These systems, however, require manipulation of a plurality of actuators to secure the rod-receiving device, shift the spinal rod, and lock the cap to the rod-receiving device. For instance, a first actuator may secure the instrument to the assembly while a second actuator drives the locking cap into the coupling assembly and a third actuator locks the locking cap to the assembly via rotation. Furthermore, the actuators in those systems often rely on threaded drive members to cause shifting of components, so that a member must be rotated a great number of revolutions in order to effect any significant amount of linear shifting.
Some such systems also have deficiencies such as an inability to fully stabilize the rod during linear shifting, failure to provide indication of when full locking of the cap and rod-receiving device has been achieved, and failure to provide mechanisms to allow multiple predetermined stages of locking for the assembly (i.e. a first “provisional” locking stage that prevents the rod from escaping the rod-receiving device but allows for rod adjustment, followed by a “full” locking stage that immobilizes the rod with respect to the coupling device). Furthermore, many of these systems are designed specifically for use with set screws or rotating cap members, and therefore may not be compatible with other types of coupling assemblies. There remains a need, therefore, for improved instruments for locking pedicle screw assemblies that are easy to use and allow a surgeon to quickly secure a spinal rod in place.